This invention relates to stainless steel and is more particularly directed to the provision of stainless Steels having superior machinability, and to methods of producing such steels.
Although much work has been done toward the development of so-called free-machining stainless steel, and although steels so classified have been marketed with some success for a number of years, the machining properties have tended to fall short of those attainable in plain carbon steels designed for good machinability, and there appears to have been a failure of understanding or full appreciation of the nature of the problems presented by stainless steel in this area. Indeed, requirements and conditions can be considered more critical than in the low or medium carbon grades of ordinary steel, in that: machined products of stainless steel, which is more costly, are often expected to have a much better finish and dimensional control than carbon steel products, while secondary operations such as reaming, threading, tapping and grinding on stainless grades can be very uneconomical at high volumes of production, as in shops using automatic screw machines; and stainless steels are inherently unsatisfactory to machine not only because of general cutting difficulties but especially because their thermal diffusivity is about two-thirds to one-half that of plain carbon steel, causing higher tool chip temperature and consequently shorter tool life.
Particular stainless steels that have heretofore been produced with special characteristics of machinability include AISI type 303 of the austenitic 300 series, and AISI type 416 from the martensitic grades of the 400 series, as well as others, e.g. the ferritic type 430F. Although the principles of the present invention are primarily illustrated with the so-called free-machining varieties such as the 303 and 416 stainless steels, the same principles are deemed applicable to a broad range of stainless steels. embracing the 200, 300 and 400 series, and also the precipitation hardening grades of both the martensitic and semiaustenitic types, such broader range being inclusive of steels for which the end service properties might restrict the usages of free-machining additive to low levels. As explained below, the invention has been developed with special applicability to the presently recognized machining grades, for example types 303 and 416, and corresponding particular objects are to provide improvement in such grades, notably in suiting the properties of these steels to the requirements of machining operations in industrial production.
It has been known that the addition or intentional inclusion of one or more elements such as sulfur, selenium, tellurium, lead and bismuth can be beneficial to machining properties in essentially all kinds of steel, and such additions of sulfur or selenium, for instance, in the amounts Of 0.15% or more have been employed in stainless types 303, 416 and 430F to provide the basis for rating such types as machinable. In a produced steel sulfur additions usually appear as sulfide inclusions, basically as manganese sulfide and these inclusions can exhibit a variety of morphology and may contain one or more other elements, such as the so-called transition elements, dissolved therein, all depending on a variety of compositional and processing factors that have not heretofore been fully elucidated.
Various and not entirely consistent views have been expressed as to the circumstances for enhancement of machinability by the use of sulfur or for assurance of a suposedly effective kind of inclusion. Thus for example, some investigators have proposed, chiefly on the basis of the ratio of manganese to sulfur in the steel (i.e., between the total amounts of these elements, in weight percent), that improvement in machinability of stainless steels can be effected by reliance on a supposedly proper range of Mn to S ratios alone. In other cases, one or another of various considerations, chiefly of the nature of specific additions or omissions of chemistry, have been asserted to be useful, but the extended investigations upon which the present invention is predicated have revealed that there has heretofore been a failure to understand or recognize many underlying factors, bearing on the nature and effectiveness of sulfide inclusions, or what inclusions are truly suitable for achieving machinability or how they can be achieved, notably in production type quantities of steel, or what may be the effect or significance of steelmaking processes on machinability and on additions supposed to improve it.
It appears, moreover, that past studies have in general failed to take proper account of various requirements of machinability, particularly the needs of industry in making machined articles from stainless steel. Thus in some cases sole reliance has been placed on limited drilling tests, such for example as in measuring the time required for a given penetration by a specified drill under a constant load, but these determinations have given little or no indication of performance in regard to surface finish, tool wear, tool life, or chip characteristics, or indeed in practical productivity, e.g. the speeds and feeds that can be used in production machinery. Attention has, of course, been given to one or another of these factors in other discussions of free machining steel, but their collective significance has not been emphasized. More importantly, in the testing or design of new steel compositions there appears to have been essentially no recognition, and certainly no report of systematic use, of production-type studies such as involve, for instance, a continuous run of several hours of an automatic screw maching producing 1,000 to 1,500 or more pieces of the test steel, each subjected to a machining cycle which includes major machining operations that reveal the performance factors mentioned above and which is representative of the kind of work required by industrial users.
Definitive information on essentially all factors of machining performance is obtainable with production-simulating tests of this sort, but they can be usefully supplemented or extended by specific single-purpose tests of rigorous design, such as turning tests for tool life and tool wear, plunge cutting for surface finish determinations, and drilling tests using suitably large drills for chip breakability determinations. Investigation has indicated that selection of compositional ranges and other characteristics to provide supposedly machinable steels, based only on one or a few limited tests such as drillability used by many of the previous investigators, can be very unreliable, and in particular do not afford good correlation with true and complete requirements for machining stainless steel, i.e., requirements as outlined above that must be met in industrial practice.
Accordingly, important aims of the present invention are to afford improved stainless steels, and methods of producing them, which have distinctly superior machining properties and which in presently preferred embodiments are well suited to the needs, in quality and production rate, of manufacturers of machined products. A further object is to provide such steels and such methods in an economical manner, and without significatntly altering the other desired properties that characterize the grade of steel to which the invention is applied.